(1) Field of the Invention
The present invention relates to a fixing device that uses an electromagnetic induction heating technology, and an image forming apparatus having the fixing device.
(2) Description of the Related Art
A fixing device provided in image forming apparatuses such as printers forms a fixing nip by pressing a heating roller or a heating belt and a pressure roller or a pressure belt against each other. Such a fixing device generally fixes a toner image formed on a recording sheet by applying heat and pressure to the toner image while the recording sheet is passing through the fixing nip with the heating roller or the heating belt heated by the heat source.
Conventionally, a halogen heater is commonly used as the heat source for the fixing device. However, in recent years, an electromagnetic induction heating technology has been attracting attention as it realizes more rapid and efficient heating than with a halogen heater and leads to energy saving. One example of the electromagnetic induction heating technology uses a heating belt having an electromagnetic induction heating layer, which is to be caused to generate heat with an electromagnetic induction coil provided outside the area in which the heating belt rotates.
A fixing device having a heating belt that uses the electromagnetic induction heating technology is provided with an induction heating power source circuit. The induction heating power source circuit is used for converting an alternating current for commercial use into a high-frequency current that resonates with an electromagnetic induction coil by using a capacitor. A high-frequency current output from the induction heating power source circuit is supplied to the electromagnetic induction coil, which causes the heating layer provided on the heating belt to generate heat. The induction heating power source circuit converts an input alternating current (50 Hz or 60 Hz) to a high-frequency current by rectifying the alternating current and performing switching with use of a switching device. Such a fixing device is capable of minimizing the thermal capacity and securing a desired temperature-rising characteristic.
A Patent Document 1 (Japanese Patent Application Publication No. 2002-237377) discloses a structure of a fixing device using the electromagnetic induction heating technology, conceived for preventing the occurrence of an ineffective current due to the phase difference between a commercial-use alternating current and a consumption current. To fulfill the purpose, the fixing device controls the turn-on time of the switching device based on a current obtained from a voltage detected between the electromagnetic induction coil and the switching device.
According to the structure disclosed in the Patent Document 1, the fixing device directly uses a pulsating current resultant from rectification of an alternating current having a frequency of 50 Hz or 60 Hz. That is, the induction heating power source circuit is not provided with a smoothing capacitor for suppressing harmonics contained in the current input to the induction heating power source circuit. Thus, the high-frequency current to be output has a frequency of 100 Hz or 120 Hz, and is subject to significant fluctuations in electrical power. Furthermore, the high-frequency electrical power is to be output with a low power factor because it is to be applied to a resonant circuit including an electromagnetic induction coil (i.e. inductor) and a resonant capacitor. This means that it is not easy to accurately detect the electrical power output from the induction heating power source.
To enable the induction heating power source to accurately output high-frequency electrical power having a predetermined value, the high-frequency electrical power is detected from the electrical power based on the alternating current input to the induction heating power source. FIG. 13 shows an example of such an induction heating power source. The induction heating power source 80 is configured as follows: first, a rectifying circuit 81 rectifies an input alternating current; second, an power converting circuit 82 converts the electrical power of a direct current resultant from the rectification to a predetermined power level; then a smoothing circuit 83 smoothes the direct current; and finally, an electrical power detecting circuit 84 detects the value of the electrical power based on the smoothed current.
For example, when the alternating current input to the induction heating power source 80 has a frequency of 50 Hz (20 ms per cycle), the electrical power of the direct current, resultant from full-wave rectification by the rectifying circuit 81, is converted by the power converting circuit 82 to be at a predetermined power level, as shown in FIG. 14A. Through the full-wave rectification by the rectifying circuit 81, the negative half of the sine wave of the input alternating current is converted to have positive polarity. After the power conversion, the direct current is smoothed by the smoothing circuit 83 to have a predetermined output level, as shown in FIG. 14B. The electrical power detecting circuit 84 detects the output level after the smoothing.
With the induction heating power source 80 having such a structure, the smoothing circuit 83 might not be able to perform sufficient smoothing when its time constant is small. Asa result, the output electrical power might fluctuate, and the detection accuracy of the electric power detecting circuit 84 might degrade. Although it is possible to suppress the fluctuation of the output electrical power from the smoothing circuit 83 by increasing the time constant of the smoothing circuit 83, this also increases the time required for the smoothing. As a result, the period from the inputting of the alternating current into the induction heating power source 80 to the detection of the power level by the electrical power detecting circuit 84 will be increased. This is problematic.
Besides, in detection by the electrical power detecting circuit 84 of one cycle (e.g. 20 ms) of the alternating current input to the induction heating power source 80, it is common that a delay due to a capacitor on the electrical power detecting circuit 84 is taken into consideration. Thus, for example, the electrical power detecting circuit 84 detects the output power with a delay of at least 10 ms. This also increases the time required for the power detection.
For use with a fixing device of an image forming apparatus, a heating belt is more preferable than a heating roller, because a heating belt has a lower thermal capacity and a better temperature-rising characteristic. However, when a heating belt having a low thermal capacity is used, the temperature greatly changes in response to changes of the electrical power. Thus it is necessary to quickly control the electrical power applied to the electromagnetic induction coil in response to the changes of the fixing temperature of the heating belt. In view of this, high-speed control of the electrical power has been conventionally performed with use of a temperature detecting device that is capable of detecting the temperature in a short time. However, even with use of such a temperature detecting device, response delay of the electrical power control might occur if the detection of the power takes a long time. As a result, it might become impossible to quickly set the temperature of the heating belt to be a desired fixing temperature.